Several attempts to enhance bony ingrowth around an implant have been described in recent years. For example Rubin et. al. in a 1994 paper entitled "Promotion of Bony Ingrowth by Frequency-Specific, Low Amplitude Mechanical Strain" published in Clin. Ortho. Rel. Res. vol. 298, pp. 165-74 demonstrated that a 20 Hz, 150 microstrain mechanical loading to the bone surrounding a turkey ulnar implant (100 seconds a day) provided significant enhancement of fixation by increased bone growth into and apposition on the implant. Such mechanical loading has also been shown to be of benefit in the healing of bone fractures.
In another study on bone ingrowth by Tanzer et. al. in a 1996 paper entitled "Effect of Non-Invasive Low Intensity Ultrasound on Bone Growth in Porous Coated Implants" published in the J. Ortho Res. Vol. 14, No. 6, pp. 901-906, using an ultrasonic mechanical input demonstrated, that low intensity ultrasound (200 microsec of 1.5 MHZ sine at 10 kHz), externally applied for 20 minutes a day in a canine implant model, promoted bone ingrowth. This has also been shown clinically to promote fracture healing. Methods to increase bone incorporation (rate and amount) such as cellular grafts, electromagnetic stimulation, Ca--P coatings and growth factors have also been used clinically with various degrees of success.
Several patents also disclose methods and apparatus for bone growth stimulation. Ryaby et al. in U.S. Pat. No. 4,315,503 uses external inductively coupled electromagnetic fields to heal pseudoarthroses, and delayed and non-union bone fractures. Ryaby teaches the importance of using specific signal waveforms to ensure the effectiveness of the technique. The patent is based upon the application of asymmetric periodic pulsed waveforms, whose choice is motivated by the endogenous electrical signals in bone induced by external strains. Another patent by Liboff et al., U.S. Pat. No. 4,932,951 teaches apparatus and method to heal bone fractures using inductively coupled low frequency sinusoidal electromagnetic signals. The specific frequency is based on a "cyclotron-resonance" condition, which is supposed to affect the trajectory of specific cations, for example, calcium. It is hypothesized that modulation of cation motion can stimulate the formation of bone. In a related patent U.S. Pat. No. 4,993,413 a method and apparatus is taught to prevent osteoporosis and to enhance new bone formation using a low frequency inductively coupled sinusoidal signal in the frequency range 15 and 75 Hertz.
A method and device are also described in U.S. Pat. Nos. 4,928,959 and 5,046,484 for increasing the amount, strength and proper anatomical distribution of bone in a patient suffering from a bone disorder based upon the use of mechanical stimuli to stimulate bone growth. These patents involve the use of an impulsive or impact force in which the subject experiences a force similar to that obtained in normal walking. In yet another U.S. Pat. No. 5,103,806 to McLeod et al., a method is described for preventing osteopenia by subjecting the subject to a sinusoidal mechanical stimulation signal in the frequency range of 10 and 100 Hertz. The patent points out the advantage of using "a relatively high frequency" in that it subjects the patient to less physical trauma compared with the impact approach taught by Bassett et al.
An excellent review of methods proposed to stimulate bone ingrowth around artificial implants (known as "osteointegration") has been published in 1993 in the Journal of Bone and Mineral Research in vol. 8, Supplement 2 entitled "Strategies to Affect Bone Remodeling: Osteointegration" by Racquel Z. Legeros and Ronald G. Craig. These methods include biomechanical, biological and biomaterial factors which affect integration of the implant into the endogenous bone.
In all of the above prior art, there is either a lack of effectiveness to enhance osteointegration and implant stability and/or a shortcoming with respect to the means available to apply biomechanical stimuli appropriate to the implant bone interface. The present invention addresses both of these shortcomings.
An interesting paper authored by Chavez et al. entitled "Assessment of oral implant mobility" published in the Journal of Prosthetic Dentistry, vol. 70, number 5, 1993 challenges the commonly accepted notion that dental implants must be totally immobile to be successful. Using a very sensitive Periotest device, the authors demonstrated that clinically successful endosseous implants could be displaced from 0.038 mm to 0.113 mm with a mean of 0.66 mm (66 .mu.m). Micromotion in accordance with the present invention is based upon amplitude displacement between 0.1 .mu.m, to 20 .mu.m.
It is also noteworthy that three papers (Holcolm et al. "Biomagnetics in the treatment of human pain: past, present, future in Environ Med, 1991a; 8:24-30, McLean et al. "Blockade of sensory neuron action potentials by a static magnetic field in the 10 mT range" in Bioelectromagnetics 1995; 16:20-32, and Cavopol et al. "Measurement and analysis of static magnetic fields that block action potentials in cultured neurons" in Bioelectromagnetics 1995; 16:197-206) demonstrate that static magnetic fields are capable of reducing pain. Additional corroborating evidence already exists in the clinical orthodontic literature.